1. Field of the Invention
This invention is related to a mono-component developer and a method for forming an image, more particularly, it is related to a mono-component developer and a method for forming a (multicolor) image comprising the steps of: forming a thin layer of the developer on a developer-holding member by making use of said mono-component developer, carrying said thin layer to a developing region, and developing an electrostatic latent image on a latent image-holding member.
2. Description of the Related Art
In recent years, an electrophotographic dry developing process has been used in not only an electrostatic copying apparatus, but also in a printer, facsimile and copying apparatus as well as combined apparatus of copying apparatus with facsimile. It has been recently required that these apparatuses are formed in lighter and smaller sizes and satisfy ecological requirements such as economy in energy, recycling, etc. In order to satisfy these requirements, an improvement and development have been extended in a method of forming an image and developer used therefore. As a dry development process in an electrostatic copying system practically used at the present are known two kinds of development, one is two component development making use of toner and carrier such as iron powder etc. and the other is mono-component development which dose not use a carrier.
The two-component development is the most widely utilized process. This process, however, suffers from the defect that it can not maintain long term image quality because of deterioration of developer. The toner particles adhere to the surface of the carrier. This process also has a disadvantage in that large developing apparatus is necessary, since it requires a control system for toner concentration to keep the concentration of the toner in the developer constant and a mixing apparatus for mixing the newly added toner with the developer. Therefore, the demand for a mono-component development system which is able to make the developing apparatus light and small has increased.
The mono-component toner development system is classified into two types of processes, one is a magnetic mono-component development system using magnetic toner, and the other is a non-magnetic mono-component development system using non-magnetic toner. In the magnetic mono-component development system, the magnetic toner is retained by making use of a developer-holding member such as a magnet etc. equipped with a means for generating a magnetic field therein and development is carried out. The magnetic mono-component development possesses several advantages that the toner may be easily carried and controlled and internal adhesion of copying apparatus, printer, etc. is minimal. However, the magnetic toner used in the magnetic mono-component development suffers from a serious disadvantage in that it cannot be full-colored because it contains black or brown magnetite and the like therein. Further, there are limits to how much the magnetic mono-component developing apparatus can be reduced in size because the developing roll must include a magnet therein, and therefore, a developing roll of a certain size is required.
On the other hand, colorization may be feasible in a non-magnetic mono-component development system because a magnetic substance is not used in the toner. And since no magnet is used in the developer-holding member, reductions in weight and size, cost-saving, etc. are feasible. In recent years, the non-magnetic mono-component development system has been, accordingly, undertaken to be practically utilized as a small-sized full-color printer.
However, in the non-magnetic mono-component development system, the toner must be supplied and held stably on the developer-holding member and charged and developed by means of only static electric force due to the absence of stable charging and carrying means. While, the two-component development system contains a stable charging and carrying material as carrier, the magnetic mono-component development system has a magnetic force of a magnetic roll as stable charging and carrying means. Therefore, the non-magnetic mono-component development system is significantly inferior to the two-component development system or magnetic mono-component development system in maintaining high image quality over a long period of time. Particularly, in full-color copying apparatus and/or printers using four color developments of yellow, magenta, cyan and black, accurate control of developing amount is required and the toner should have strict performances such as speedy and uniform charge, good fluidity, etc. in order to cope with the miniaturization/speeding up of recent years.
In recent years, instead of corona discharge a method for applying voltage to the surface of a photosensitive material in contact with a charging material directly or via a recording material or while pressing the surface of said photosensitive material directly by the charging material or via the recording material to charge and transfer in direct as a means for charging uniformly the surface of the photosensitive material (electrostatic charged image-holding member) or for transferring a toner image on said photosensitive material as described in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-149669 and 02-123385 has been growing in popularity. The method for charging and transferring is superior to conventional corona discharge, since no ozone is produced and environmental resistance properties are excellent. In this method, the shear in transfer is reduced, since the surface of the photosensitive sheet holding an image is in contact with a transfer sheet(paper). Furthermore, the mono-component developing apparatus, especially the non-magnetic mono-component developing apparatus has the advantage of adapting well to shortening of the path for carrying the transfer sheet and to reduction of the diameter of the photosensitive material which result from miniaturization of image-forming apparatus required by the mono-component developing apparatus or non-magnetic mono-component developing apparatus.
However, this apparatus also has disadvantages. The transfer apparatus must have pressurization to a certain extent during the transfer step. The toner image formed on the electrostatic latent image-holding member is also effected by this pressurization and agglomeration tends to occur, so that a phenomenon of inhibition of migration of the toner to transfer material is seen. This phenomenon is particularly significant in a linear portion in the range of 0.1 to 2.5 mm. The inventors of the present invention inferred that a good deal of toner is applied to the liner portion due to the edge effect and that agglomeration occurs through pressure. The toner image (transfer image) formed at this time has the serious defect of being formed only in profile, the so called "hollow character". In this way, the mono-component developing apparatus, especially the non-magnetic mono-component developing apparatus requires a toner exhibiting prevention of "hollow character".
In order to stabilize the charging and carriage of the toner, a charge controlling agent is conventionally added. Typical examples of negative charge controlling agents may include metal-containing azo dyes, and metal-containing salicylic acid compounds. Typical examples of positive charge controlling agent may include quaternary ammonium salts etc. An addition of such charge controlling agents is effective and indispensable in the maintenance of speedy and uniform charging of toner over a long period of time. However, such charge controlling agents may fail to exert fully their effects due to the presence of additives used in combination therewith, especially externally supplied additives of fine particles for imparting fluidity.
For example, inorganic fine powders like, for example, silica etc. are added to toners. However, this method can not make the carriage of the toner to the developer-holding member and chargeability of the toner optimum at either high temperature and high humidity or at low temperature and low humidity, and has disadvantages in that poor reproducibility of image density, background fogging, dripping of the toner, and internal adhesion of the apparatus, etc. occur.
In order to reduce such disadvantages, use of surface treated inorganic fine powder has been proposed. For example, in JP-A Nos. 46-5782, 48-47345, 48-47346, 59-34539, 59-198470, 59-231550, etc. is described a hydrophobic treatment of silica fine particles. However, these inorganic fine particles can not exert satisfactory effects, especially in chargeability. In the case of using a polyester resin as the binding resin, absolutely no effect was obtained. Further, the above problem of the hollow character occurred in the contact-transfer system.
A method for treating the surface of the inorganic fine particle with silicone oil is proposed as described in, for example, JP-A Nos. 61-249059, 61-277964, etc. as a method for increasing the hydrophobic nature of the inorganic fine particle. It is known that a significant effect can be obtained through this method because of the low surface tension peculiar to silicone oil and because of the lowering of non-electrostatic adhesion between toners, and especially because of the lowering of toner agglomeration under pressure.
However, this conventional method exerts significant effects on the hollow character, but dose not solve the above problem of chargeability (environmental dependency). As a method of relaxation of the negative chargeability of the toner particles, a method of external addition of silica fine particles surface-treated with an amino-denatured silicone oil (JP-A No. 64-73354) and a method of external addition of silica fine particles surface-treated with aminosilane and/or amino-denatured silicone oil have been proposed (JP-A No. 1-237561). These methods could not, however, solve the environmental dependency problem.
Further, the non-magnetic mono-component toner should, as above described, supply and hold stably the toner on the developer-holding member by only electrostatic force and should be charged and developed. Therefore, the non-magnetic mono-component toner must be charged by friction of contact with the developer-holding member (sleeve) and the charging blade for a short time and in a small space. Therefore, the toner must be charged quickly. However, the toner to which the silica fine particles are externally added can not usually be charged quickly with edge, whether with a two-component toner or a magnetic mono-component toner. The non-magnetic mono-component toner has the disadvantages of reverse pole fogging and toner clouding (internal adhesion of the apparatus) tending to occur easily at the low temperature and low humidity at which high charge can be obtained.
As above stated, even if the silica fine particles are subjected to hydrophobic treatment, treatment for relaxation of negative chargeability, etc. the environmental dependency of the charge, charging speed and lack of charge distribution can not be improved with only the silica fine particles.
Titania can be chosen as an inorganic oxide added for the purposes of charge and fluidity. The titania usually used can be charged more quickly than silica and may presumably make the charging distribution sharp through its low resistance. However, in case where titania is added high charge can not be given to the toner and a lowering of the amount carried, lowering of reproducibility of density caused by a lowering of charge and fogging of background tend to occur more easily.
In order to improve this chargeability, whether in two-component or mono-component systems, is proposed a method for externally adding to the toner a hydrophobic titanium oxide obtained by treating the surface thereof with a silane compound, a silane coupling agent, and a silicone oil, etc. (JP-A Nos. 58-216252, 60-123862, 60-238847). Using this conventional method, the charging level of mono-component toner can be improved to a certain extent depending on the types and amounts of treating agents used. Particularly when a treatment is carried out with silicone oil, no phenomenon of hollow character occurs in the contact-transfer system. However, neither charging level nor environmental dependency can be improved satisfactorily.
Improvements in charging level and environmental dependency can be seen by increasing the hydrophobic property of the titanium oxide with a processing agent. However, this titanium oxide is significantly inferior to conventional titanium oxide with regard to its charging speed, sharpness of charging distribution, etc. after the hydrophobic treatment.
The crystals of titanium oxide can be obtained by sulfuric acid method or hydrochloric acid method from an ilmeniteore. However, when using these methods, chemical bonds formed by dehydration-condensation are naturally present in the crystal obtained. It was not easy to redisperse such agglomerated particles. Using conventional techniques this is because the derived titanium oxide forms secondary and tertiary agglomerations. The fluidity increasing effect of the toner was also significantly inferior to that of silica. On the other hand, the titanium oxide conventionally used has a specific gravity larger than that of silica and has a disadvantage in that it can not tightly adhere to the surface of the toner. It is easily removed from the surface of the toner. Therefore, the titanium oxide is inferior to silica with regard to the charge stability over a long period of time resulting from sleeve adhesion. Titanium oxide also tends to cause adhesion of photosensitive bodies, and thus deterioration of and defects in image quality.
In order to achieve compatibility of fluidity improvement with environmental dependency of charging is attempted an addition of hydrophobic titanium oxide in combination with hydrophobic silica (JP-A No. 60-136755). While the defects of each hydrophobic titanium oxide and hydrophobic silica may be temporarily depressed by this method, they are much more subject to the influence of the other additive depending upon the state of dispersion. It is difficult to control stably the defects of each hydrophobic titanium oxide and hydrophobic silica over a long period of time.
A method is proposed for adding hydrophobic amorphous titanium oxide obtained by hydrolysis to the toner (JP-A Nos. 5-204183 and 5-72797). However, while titanium oxide can improve both charging performance and fluidity of toner, much water is absorbed and contained in the particles, which remains in the photosensitive material at the time of transfer. That is to say, since adhesion between the amorphous titanium oxide and the photosensitive material is strong, only the amorphous remains on the photosensitive material without being transferred. This causes white spots on images or the photosensitive material is damaged by hard titanium oxide at the time of cleaning.
On the other hand, in a method for purifying the titanium oxide by wet process, a method is proposed for hydrolyzing a silane compound in an aqueous medium, treating the surface of the titanium oxide, taking out the titanium oxide in a state of depressed agglomeration and adding the titanium oxide thus obtained to the toner (JP-A No. 5-188633).
The titanium oxide obtained by this method can improve the fluidity of the toner more than the titanium oxide obtained by conventional hydrophobic process can, however, it does not satisfy high negative chargeability and environmental dependency. It also adversely affects the charging speed (admixing property of additional toner) and charging distribution.
When these inorganic oxides are added to the surface of the toner, a filming or fusing of toner to the layer forming material occurs because of the stress applied to the toner from the layer forming material etc. by continuous use over a long period of time, or a change in the toner chargeability occurs by removal or embedding of additives externally supplied. Thus, this conventional method can not maintain stable charging and carriage of the toner over a long period of time easily. In order to solve these problem, is proposed a use of a specific binding resin for prevention of embedding of the additives externally supplied in, for example, JP-A Nos. 6-95429, 6-102699, 6-266156, etc. And a use of specific charging-controlling agent and external additives is proposed in JP-A Nos. 6-51561, 6-208242, 6-250442, etc. However, these methods do not exert sufficient effects. Particularly, in a full-color development system in which four colors are superinposed upon each other, the amount of developing toner development must be controlled very accurately. Therefore, there still remain unsolved problems with regard to the charging amount and long term-stabilization of carriage amount of the toner.
At present, the methods for forming an image adopted in a full-color copying apparatus or printer making use of electrophotographic systems, are exemplified by a-1) a system in which four developing apparatuses are arranged around the photosensitive material and steps of charging, exposure and development are repeated with respect to each toner in four cycles, and a-2) a system in which the charging, exposure and development of four color toner are carried out in one cycle, as well as a-3) a system in which four apparatuses of each charging apparatus, exposure apparatus, developing apparatus and transfer apparatus are disposed in one single apparatus and the toner images are superimposed.
As the system in which four color toners are superinposed are exemplified by b-1) a system in which a toner image formed on the photosensitive material is transferred and superimposed color by color onto a transfer drum around which transfer paper is wound, b-2) a system in which a toner image formed on the photosensitive material are transferred to an intermediate transfer sheet, and color toner images are superimposed on to the transfer sheet and then transferred collectively to the transfer sheet, and b-3) a system in which color toner images are superinposed on to the photosensitive material, and then transferred collectively to the transfer sheet.
While these methods have advantages and disadvantages in printing speed, apparatus size, etc., they are at present utilized according to the objects of the users.
The above systems b-2) and b-3) have the following advantages; (1) system b-2) has good transfer paper carrying properties; and (2) system b-3) is capable of being reduced in size because use of an intermediate sheet is unnecessary. However, these two methods have the disadvantage that transfer of toner images formed on the bottom layer of the transfer paper is difficult because the toner images ultimately transferred to the transfer paper become multilayered.
The full-color development has problems in common that the chargeability of the toner varies over repeated use, and thus the tone of the image changes. However, there has been no method for forming multicolor images able to simultaneously satisfy the problems.
In order to improve the transferring property of the above superimposed toner image in addition to stabilizing the above charging amount, it is necessary that a reduction in adhesion between the toner and the photosensitive material and a reduction in adhesion between the toner and intermediate transfer sheet are made.